Question

How do you name this compound that has six carbons and two `"CH"_3` on the third carbon from the right? The rest are `"H"`'s

Answer 1

In any straight-chained hydrocarbon, you'll want to look for the longest chain.

Look at it for a while and see if you can spot that the longest chain is six carbons:

`color(white)(..................................)"CH"_3`
`color(white)(..................................)|`
`color(red)("H"_3"C"-"H"_2"C"-"H"_2"C"-"C"-"CH"_2-"CH"_3)`
`color(white)(..................................)|`
`color(white)(..................................)"CH"_3`

So the parent compound is hexane, `"C"_6"H"_14`. Next, you'll want to find the smallest carbon index relative to one end of the longest chain.

• Starting from the left end would give you carbon-4.
• Starting from the right end would give you carbon-3. You want this one, then.

Lastly, you'll want to know what to call the `-"CH"_3` groups that are on carbon-3, and use prefixes to denote any duplicates (due to many same groups, not due to multiple groups on the same carbon).

Those are methyl groups, fragments of methane (`"CH"_4`). Two of them on the main chain means a prefix of di-.

So, put it together.

1. The comma-separated carbon indices at which the branched groups are placed go first, connecting to which functional groups by a hyphen. Place these in alphabetical order by stem (in this example there are no exceptions).
2. Duplicate functional groups use prefixes to denote how many of them there are.
3. For simple alkanes, the parent compound name goes last.

`stackrel("carbon indices")(overbrace(3,3"-")^("occupied")) + overbrace("di")^"prefix" + overbrace("methyl")^("stem") + stackrel("number of carbons")(overbrace("hex")^("in longest chain")) + overbrace("ane")^"parent compound"`

`=> ulbb(3,3"-dimethylhexane")`

Common misnamings are:

• 4,4-dimethylhexane
• 2-ethyl-2-methylpentane
• 2-methyl-2-propylbutane

The first one is due to counting from the wrong end and not minimizing the carbon index. The second one is from identifying the longest chain as simply straight across. The third one is from identifying the longest chain as simply straight down.